The present invention generally relates to high frequency, high data rate wireless telecommunication systems and, more particularly, to band efficient modulation for quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM) using monolithic microwave integrated circuits (MMIC).
Capability of current communication systems is limited due to smaller channel bandwidth allocations, typically less than 100 MHz (mega Hertz, or million cycles per second), available in the FCC allocated frequency bands in the range of 1 to 40 giga Hertz (GHz). One way for current systems to achieve higher data rates within these bands, is to use modulation schemes which are more bandwidth efficient, but achieving more bandwidth-efficient modulation presents rigorous technical challenges and typically requires more complex and expensive hardware.
Conventional modulation systems consist of a modulator operated at an intermediate frequency (IF) and a number of filters, amplifiers and mixers that upconvert the modulated signal to the transmit frequency, also called the carrier frequency. The demand for higher speed data transmission imposes increasingly stringent requirements on the modulators in these systems, as data rates approach the maximum bandwidth capabilities of these circuits. At multi-giga bit per second (Gbps) data rates, these modulator circuits become impractical as the bandwidth required for the high, multi-Gbps, data rate becomes very wide relative to the intermediate frequency, referred to as relative bandwidth requirement, and exceeds the technology limitations of the modulator circuit.
Conventional modulation systems may address the relative bandwidth requirement by upgrading to newer and faster transistor technology, such as silicon-germanium, or SiGe, heterojunction transistor technology. The relatively extensive hardware requirement for conventional modulation systems, however, makes upgrading the hardware relatively expensive. Another approach is to use digital modulation to enhance spectrum, i.e., bandwidth, efficiency. While digital techniques are generally desirable, a drawback to the digital modulation approach is the dependency on technological development of state-of-the-art processes. This dependency becomes more critical when pushing the performance capabilities of new processes. In business applications, it may be difficult and costly to qualify new processes, especially where high reliability is a prime concern.
MMICs have become an important advancement in the communications field. MMICs are low cost, high bandwidth circuit devices that operate at microwave and millimeter wave frequencies. MMIC's are analog circuits which downconvert a modulated signal to a baseband signal or upconvert the baseband signal to a microwave signal. One way to generate a high data rate signal is to directly modulate the microwave carrier at the high data rates. This direct modulation scheme reduces physical size and cost of the communications system by eliminating much of the hardware previously needed in such systems. A major technical challenge in this type of modulation, however, is achieving low amplitude and phase error over the wide bandwidth requirement.
Direct carrier modulation may use mature microwave device technology such as GaAs, and may also apply digital modulation techniques such as QPSK. An example of QPSK modulation is illustrated by the block diagram of FIG. 1, where two-bit data word 102 and carrier 104 are input to phase modulator 100, which outputs QPSK modulated carrier 106 corresponding to a signal, S, of the form:S(t)=A cos(ωst−θ+ψ)  (1)where A is the carrier amplitude constant and ψ is the phase constant. There are four possible values for two-bit data word 102. Phase modulator 100 maps each of the four possible values for two-bit data word 102 to a distinct value of the phase angle θ.
The QPSK modulated carrier 106 output from phase modulator 100 may be represented on a phase diagram such as phase diagram 200 seen in FIG. 2. Phase diagram 200 shows that phase angle θ will take on the form of one of four phases separated by 90 degrees. As shown in FIG. 2, each of the four possible values of two-bit data word 102 is represented by a symbol 202, which is a point, or vector, s1, s2, s3 or s4, in the phase plane of phase diagram 200. Two bits of information, or one symbol, is sent every word time corresponding to one of the four vectors, or symbols, in phase diagram 200.
The symbols of QPSK signals may also be conceptualized as two pairs of a bi-orthogonal set. FIG. 3 shows a common implementation, using that concept, of QPSK modulator 300 employing orthogonal bi-phase shift keying (BPSK) modulators 310 and 320. The circuit of QPSK modulator 300 shown in FIG. 3 uses double-balanced mixers for BPSK modulators 310 and 320. As seen in FIG. 3, two-bit data word 302 is extracted from bit sequences 303 and 305. Bit sequence 303 and carrier 314 are input to BPSK modulator 310, which outputs BPSK modulated signal 316. Bit sequence 305 and carrier 324 are input to BPSK modulator 320, which outputs BPSK modulated signal 326. BPSK modulated signals 316 and 326 are added by summer 330 and output as QPSK modulated carrier 336 corresponding to a signal, S, of the form:S(t)=A cos(ωst−θ+ψ)  (2)where A is the carrier amplitude constant and ψ is the phase constant. There are four possible values for two-bit data word 302 each of which is mapped to a distinct value of the phase angle θ. Because carriers 314 and 324 differ in phase by 90 degrees, phase angle θ will take on one of four phase values separated by 90 degrees, as shown in FIG. 2, with each of the four possible values of two-bit data word 302 represented by a symbol 202, which is a vector, s1, s2, s3 or s4, in the phase plane of phase diagram 200.
FIG. 4 shows a basic MMIC implementation of QPSK modulator 400 using a number of 90 degree Lange couplers 402, 404, 406, and 408. Conventional MMIC implementation of QPSK modulator 400 has carrier input 410 entering QPSK modulator 400 and then split into two orthogonal components 412 and 414 by use of Lange coupler 402. Orthogonal components 412 and 414 then pass into two variable bi-phase modulators 404 and 406, which each include another Lange coupler. Bi-phase modulators 404 and 406 split the signal again before the signal goes into variable reflection loads 416, 418, 420, and 422. These complex circuits have been popular as analog modulators, or as vector modulators, where both amplitude and phase are varied. Normally, 90 degree hybrids or Lange couplers are used. The added complexity of these circuit designs and the technical difficulties they present in achieving the low amplitude and phase error over the wide bandwidth requirement needed for high data rate operation, however, has largely limited them to Mbps data rates, where the relative bandwidth requirement is reduced because the bandwidth required for the lower, Mbps, data rate is narrow enough relative to the higher intermediate or carrier frequency.
FIG. 5 shows how two QPSK modulation systems 510 and 520 may be combined in a QAM modulation system 500 to achieve a 16 QAM signal 536. An RF or IF carrier is provided by local oscillator 504 using timing reference 501, as known in the art. The RF or IF carrier is split into carriers 514 and 524, and each is fed into QPSK modulation systems 510 and 520, respectively. Two-bit data word 512, which includes bits b0 and b1 as shown in FIG. 5, and carrier 514 are input to QPSK modulation system 510. QPSK modulation system 510 outputs QPSK modulated carrier 516 corresponding to a signal which may be represented, as described above in connection with FIG. 2, by vectors 616 on phase diagram 610 shown in FIG. 6. Similarly, two-bit data word 522, which includes bits b2 and b3 as shown in FIG. 5, and carrier 524 are input to QPSK modulation system 520. QPSK modulation system 520 outputs a QPSK modulated carrier 526, which travels through attenuator 527. Attenuator 527 lowers the amplitude of QPSK modulated carrier 526. The attenuated QPSK modulated carrier 526 corresponds to a signal which may be represented, as described above in connection with FIG. 2, by vectors 626 on phase diagram 620 shown in FIG. 6.
As seen in FIG. 5, the two QPSK modulated carriers 516 and 526 are added by summer 530 and output as QAM modulated carrier 536 corresponding to a signal which may be represented, as described above in connection with FIG. 2, by vectors 636 on phase diagram 630 shown in FIG. 6. The addition of QPSK modulated carriers 516 and 526 is indicated in FIG. 6 by plus sign 632 and equal sign 634 representing addition of phase diagrams 610 and 620 corresponding to QPSK modulated carriers 516 and 526, respectively. Because each vector 616 and 626 represents a signal, addition of the phase diagrams is accomplished by adding each possible pair of vectors 616 and 626 to produce a vector or symbol 636 in phase diagram 630. The configuration formed by symbols 636 is referred to as a 16 QAM constellation. The vectors 616 are also shown in phase diagram 630 to provide a size orientation for the purposes of illustration only, but do not form part of the 16 QAM constellation illustrated in phase diagram 630. Each symbol 636 represents a pair of two-bit data words 512 and 522, which may be viewed as a four-bit data word, b0, b1, b2, b3. Each four-bit data word has 16 possible values each of which is mapped by QAM modulation system 500 to one distinct symbol 636 of the 16 symbols 636.
Physical limitations and variances in the circuits used to implement QAM modulation system 500 cause variance, or inexactitude, in the amplitudes and phases of symbols 636 during transmission of the QAM modulated signal. The variances may cause some of the symbols 636 to occasionally be transmitted closer together in phase diagram 630. If the variances, or imperfections, are too great, the receiver may not be able to reliably provide resolution of the symbols to permit faithful demodulation of the QAM modulated signal at the receiver. Imperfections that limit the effectiveness of QAM modulation systems, may be reduced by achieving low amplitude and phase error over the channel bandwidth. The technical difficulties in achieving low amplitude and phase error over the channel bandwidth have restricted the use of QAM modulation systems to lower frequencies and narrower bandwidths than is desirable.
As can be seen, there is a need for direct modulation in communication systems that reduces the relative bandwidth requirement, thus allowing higher data rates relative to the carrier frequency. Moreover, there is a need for modulation in communication systems that use MMIC to improve amplitude and phase balance, and achieve operation at higher frequencies where wider channel bandwidth allocations are potentially available.